The ultimate aim of radiation treatment is to deliver a prescribed dose to a well-delineated tumour volume and, at the same time, minimize the radiation dose to the surrounding normal tissues. Verification of the three-dimensional dose distribution around the tumour before treatment is given to a patient is essential to ensure the above aim is achieved. A phantom incorporated with a dose-recording medium is usually used as a surrogate for the patient's treatment for verification.
Of all the dose-recording media used, radiocbromic films are gaining popularity in dose verification work owing to their advantages over the conventional media. Radiochromic films are films which change from colorless to a bluish color upon irradiation by ionizing irradiation. The darkness of the blue shade is proportional to the amount of energy deposited on the films. They are insensitive to daylight. There exist two absorption peaks for the exposed films at which the most sensitive measurements can be made. Radiocbromic films are self-developing, tissue equivalent; their responses are practically energy and dose-rate independent. They can offer an extremely fine spatial resolution up to 1200 lines/mm due to their grainless nature. In conclusion, radiochromic films are increasingly being used for measuring two-dimensional dose distribution. This is particularly true in intravascular brachytherapy where detailed dosimetry data at 2-5 mm from the source axis is required for prescribing the radiation dose to a patient.
Radiochromic films need to be read out before any quantitative analysis of the absorbed dose distribution can be made. Currently available two-dimensional densitometers are not primarily designed for reading radiochromic films. There are three major problems associated with those densitometers.
Firstly, they utilise light sources including lasers, fluorescent lamps and light-emitting diodes that do not have an emission spectrum that matches the absorption spectrum of the radiochromic films. This may compromise the sensitivity of the measurement.
Secondly, the wavelength of the light source is not changeable. Transmittance measurement taken at a single wavelength suffers from the disadvantages that measuring at the major absorbing peak may saturate at high-doses whereas measuring at the minor absorbing peak may not be sensitive enough to detect low doses.
Thirdly, those densitometers can only provide a spatial resolution of less than 20 lines/mm due to the size of the light source and photodetection method used. Their low spatial resolution is much inferior to the inherent spatial resolution of 1200 lines/mm of the radiochromic films. Those densitometers therefore fail to satisfy the need for measuring dose distributions in micrometer spatial resolution.